A discussion of invasive arterial blood pressure measurement is a slippery slope into terrible gibberish. Instead, one should focus on what is expected from a viva answer. The specific things which could be asked are about the method of arterial cannulation, indications for it, sources of inaccuracy, and possibly Complications of arterial cannulation. One may wish to expand into such themes as the zero level of the arterial transducer, what relevance this has to the accuracy of measurement, and how it influences the utility of cerebral perfusion pressure as a therapeutic targetQuestion 6.2 from the second paper of 2009 briefly touches on these issues.

  • Method of invasive blood pressure monitoring
    • Arterial cannula
    • No more than 1.2m of fluid-filled compression-resistant tubing
    • Fluid column is exposed to the Wheatstone bridge strain-gauge
    • Wave form built up by Fourier analysis from sinusoids
    • Different waveform morphology is to be expected depending on the site of measurement
  • Indication for invasive haemodynamic monitoring:
    • Rapidly or dangerously fluctuating blood pressure
    • A prolonged course of blood pressure monitoring (avoids the pressure-related complications of repeated NIBPs)
    • Titration of vasoactive agents
    • Frequent blood sampling
    • Inaccuracy of non-invasive monitoring (eg. in massive obesity)
  • Sources of inaccuracy in the invasive measurement of arterial blood pressure
    • Cannula is in a damaged artery; spasm or partial obstruction
    • Cannula or tubing is kinked
    • Air bubbles or clots in the tubing
    • Inadequate frequency response of the transducer (should be at least 30Hz, and more like 100Hz)
    • Inappropriately zeroed
    • Inappropriately levelled
    • Factory-calibrated transducer came with faulty calibration.

Zeroing and levelling of the transducer

  • "Zeroing" the transducer sets the atmospheric pressure as the "0 mmHg" reference point.
  • "Levelling" the transducer sets a particular height along a fluid column as the "0 mmHg" reference point.
  • The arterial line pressure transducer should be "levelled" at the phlebostatic axis, which is a reference level we have used since probably 1945.
  • The phlebostatic axis corresponds roughly with the position of the right atrium
  • This level has generally been accepted as the ideal reference level for measure the pressure of the blood returning to the heart, and was therefore adopted as the reference level for CVP measurement.
  • For arterial pressure measurements, at least since 2001 or so we have been also levelling the arterial lines at the phlebostatic axis. Prior to that, some units levelled their arterial lines at the level of the catheter insertion site.
  • The specific reference point for the arterial transducer is actually the aortic root, but because it is very close to the right atrium the two reference levels are essentially the same.
  • For measurement of cerebral perfusion pressure, the reference level should be at the middle cranial fossa, but whether this matters or not is a subject of some debate.

References

This FRCA study document on arterial pressure monitoring is a goldmine of detailed information.

Lodato RF, Schlichting R: "Arterial pressure monitoring. Arterial catheterization: complications." In Principles and Practice of Intensive Care MonitoringVolume Part III. 2nd edition. Edited by Tobin MJ. New York: McGraw Hill; 1998::733-756.

Winsor, Travis, and George E. Burch. "Phlebostatic Axis and Phlebostatic Level, Reference Levels for Venous Pressure Measurements in Man." Experimental Biology and Medicine 58.2 (1945): 165-169.

McCann, Ulysse G., et al. "Invasive arterial bp monitoring in trauma and critical care: Effect of variable transducer level, catheter access, and patient position." CHEST Journal 120.4 (2001): 1322-1326.

Thomas, E., M. Czosnyka, and P. Hutchinson. "Calculation of cerebral perfusion pressure in the management of traumatic brain injury: joint position statement by the councils of the Neuroanaesthesia and Critical Care Society of Great Britain and Ireland (NACCS) and the Society of British Neurological Surgeons (SBNS)." British journal of anaesthesia (2015): aev233.